The present invention relates to optical fibres and especially to optical fibres having micro-structures in core and/or cladding region(s). The fibres may be utilized for dispersion compensation and non-linear applications.
The dispersion properties of conventional optical fibres are receiving a continuously high research interest in connection with high-capacity optical communication, soliton propagation, and control of non-linear effects. Accordingly, there is a strong interest in realizing new types of optical fibres that may provide new dispersion properties or may counteract some of the undesired dispersion properties of existing fibres.
Recently a new type of optical fibre that is characterized by a so-called micro structure has been proposed. Optical fibres of this type (which are referred to by several namesxe2x80x94as e.g. micro-structured fibres, photonic crystal fibre, holey fibre, and photonic bandgap fibres) have been described in a number of references, such as WO 99/64903, WO 99/64904, and Broeng et al (see Pure and Applied Optics, pp. 477-482, 1999) describing such fibres having claddings defining Photonic Band Gap (PBG) structures, and U.S. Pat. No. 5,802,236, Knight et al. (see J. Opt. Soc. Am. A, Vol. 15, No. 3, pp. 748-752, 1998), Monro et al. (see Optics Letters, Vol. 25 (4), p. 206-8, February 2000) defining fibres where the light is transmitted using modified Total Internal Reflection (TIR). This application covers fibres that may guide by both physical principles and we shall use the term micro-structured fibres to generally describe these fibres.
Micro-structured fibres are known to exhibit dispersion properties that are unattainable in conventional optical fibres (see e.g. Ranka et al., Optics Letters, Vol. 25, No. 1, pp.25-27, 2000, Broderick et al. Optics Letters, Vol. 24, No. 20, pp. 1395-1397, 1999, Mogilevtsev et al. Optics Letters, Vol. 23, No. 21, pp. 1662-1664, 1998). These properties include shifting the zero dispersion wavelength below 1.3 xcexcm. This has e.g. in the above-cited Ranka-reference been utilized for super-continuum generation of light over a very broad frequency range (covering visible to near-infrared wavelengths). The development of such white-light generators using micro-structured fibres was made possible through the design of micro-structured fibres with high anomalous waveguide dispersion at visible wavelengthxe2x80x94and it has fuelled a large research interest into non-linear effects in micro-structured fibres (Fedotov et al. JETP Letters, Vol. 71, No. 7, pp. 281-284, 2000, Wadsworth et al. CLEO 2000, Paper PD1.5, 2000). The above-cited references all describe fibres with zero dispersion wavelength shifted below 1.3 xcexcm. The fibres are characterized by a relatively high cladding air-filling fractionxe2x80x94air hole diameters, d, of more than 0.45 times the centre-to-centre distance between two nearest air holes, xcex9, and they all have a solid core. The size of the core is relatively smallxe2x80x94about 1.5 xcexcm in diameter. It is a disadvantage of the prior art fibres with zero-dispersion wavelength shifted below 1.3 xcexcm that they are not strictly single-mode at visible wavelengths, but support a few (or more) guided modes. In the above-cited reference by Ranka et al., it is demonstrated that for relatively short fibre lengths, the fundamental mode of such fibres may be considered undisturbed by any higher order guided modes (i.e. there is a low coupling coefficient between the fundamental and the higher order modes). However, for guidance over longer fibre lengths (i.e. hundred of meters) it is a disadvantage of the prior art fibres with zero dispersion wavelength shifted below 1.3 xcexcm that they are not strictly single mode at visible wavelengths. It is a further disadvantage of the prior art fibres with zero dispersion wavelength below 1.3 xcexcm that they will be highly multimode at visible wavelengths if the core size is increased above 2 xcexcm. It would be an advantage if fibres with zero dispersion wavelength shifted below 1.3 xcexcm could be realized so as to have a core that was comparable in size to that of standard transmission optical fibres (i.e. to have a core of around 5 micron in diameter).
Another important aspect of micro-structured fibres is that they may exhibit normal dispersion (or so-called negative dispersion) at near-infrared wavelengths. Fibres with large negative dispersion at 1.55 xcexcm are attractive for use as insertion-components in existing optical fibre communication links, as they may be used to compensate the positive dispersion around 1.55 xcexcm of already installed standard transmission fibres (i.e. fibres that are designed to operate in the second telecommunication window and have a zero dispersion wavelength at 1.3 xcexcm).
Monro et al. have presented micro-structured fibres having dispersion values of about xe2x88x9230 ps/nm/km at 1.55 xcexcm (see Journal of Lightwave Technology, Vol. 17, No. 6, pp. 1093-1102, 1999). The fibres presented by Monro et al. are characterized by a solid core surrounded by micro-structured cladding with a close-packed arrangement of identical air holes. The cladding holes have a size d/xcex9 around 0.2. It is a disadvantage of the fibres presented by Monro et al. that the dispersion is not more negative than xe2x88x9230 ps/nm/km. DiGiovanni et al. (see U.S. Pat. No. 5,802,236) have presented micro-structured fibres that provide significantly larger negative dispersion at near-infrared wavelengths. DiGiovanni et al. disclose micro-structured fibres that are characterized by a core and a micro-structured cladding. The cladding consists of inner and outer cladding features, thereby forming an inner and an outer cladding region. Both the inner and outer cladding of the fibres have an effective index that is lower than the core refractive index at all wavelengths. The features of the inner cladding region (preferably air holes) act to lower the effective refractive index compared to the effective refractive index of the outer cladding region. Hence, the fibres disclosed by DiGiovanni have a so-called xe2x80x9cdepressedxe2x80x9d cladding design. The use of depressed cladding regions is well-known from the development of conventional dispersion compensating fibres (see e.g., M. Monerie, Propagation in doubly clad single-mode fibres, IEEE Journal of Quantum Electronics, vol. QE-18, no. 4, April 1982, pp. 535-542). To those skilled in the art, it will be recognised that in order to increase the negative dispersion of the fibres disclosed by DiGiovanni et al., the size of the cladding features must be increased. Digiovanni et al. disclose fibres that have dispersion of up to xe2x88x921700 ps/nm/km. It is a disadvantage of the fibres disclosed by DiGiovanni that the depressed cladding design does not allow to increase the inner cladding feature size so as to obtain negative dispersion of more than xe2x88x922500 ps/nm/km. This latter limit of maximum obtainable negative dispersion was predicted by Birks et al. (see Photonics Technology Letters, Vol. 11, No. 6, pp. 674-676, 1999). Birks et al. studied the fundamental limits of negative dispersion that can be obtained in solid core micro-structured fibres made of pure silica and air. Birks et al. argue in the above-cited reference that by increasing the void size (air holes), the negative dispersion of micro-structured fibres is generally increased. Hence, an ideal micro-structured fibre (with respect to negative dispersion) consistsxe2x80x94according to Birks et al.xe2x80x94merely of a thin silica rod (the fibre core) surrounded by air. Hence, Birks et al. made a prediction of the maximum obtainable negative dispersion based on the study of a solid silica rod surrounded completely by air (this case corresponds to the inner cladding features of the fibres disclosed by DiGiovanni et al. being so large that they overlap each-other). For such an ideal micro-structured fibre, Birks et al. found a dispersion of xe2x88x922000 ps/nm/km. This result has been taken as the maximum obtainable negative dispersion that can be obtained using silica-based optical fibres. It is a disadvantage of the fibres disclosed by Birks et al. that a negative dispersion of more than xe2x88x922500 ps/nm/km cannot be obtained. It is a further disadvantage of the fibres disclosed by Birks et al. and of DiGiovanni et al. that the fibre core must be very small (about 1 xcexcm or less in diameter) in order to ensure single mode operation at near-infrared wavelengths while exhibiting large negative dispersion.
The present invention provides fibres that are substantially single mode at visible wavelengths while having zero dispersion shifted below 1.3 xcexcm. This application further discloses fibres that are strictly single mode, have a zero dispersion wavelength below 1.3 micron, and a core diameter of more than 2 xcexcm.
This application discloses micro-structured fibres that have dispersion significantly more negative than xe2x88x922500 ps/nm/km. The present inventors have realized that it is advantageous to turn up-side-down the usual design rules for realization of fibre with large negative dispersionxe2x80x94and to design fibres with a so-called xe2x80x9craisedxe2x80x9d, micro-structured, inner cladding region. As documented in this application, it becomes possible to realise micro-structured fibres with negative dispersion of at least up to xe2x88x924500 ps/nm/km. This application describes in detail the design-route that the present inventors have found in order to realize such fibres and discloses a number of preferred embodiments of fibres according to the present invention.
The present invention is particularly aimed at fibres for dispersion compensating and/or dispersion slope compensating applications.
While non-linearities are usually undesired for transmission and dispersion compensating fibres, they are for, other applications, desired as they may be utilized to provide different kinds of functionalities in optical fibres (such as wavelength conversion, parametric amplification, supercontinuum generation, solitons etc.). For a detailed description of non-linear fibres and their applications, please see Agrawal, xe2x80x9cNonlinear fiber opticsxe2x80x9d, Academic Press, third edition, 2001.
Also for non-linear fibres, dispersion plays an important role, and in order to realise improved non-linear fibres, it is vital to be able to control and manipulate the dispersion properties of such fibres accurately. The present invention also addresses fibres with special dispersion properties for a number of non-linear fibre applications in the near-infrared wavelength range. In particular, the present invention provides new micro-structured fibres with small cores and so-called flat, near-zero dispersion at near-infrared wavelengths (especially around 1.5 xcexcm) for use as non-linear fibres.
Ferrando et al. have studied fibres with nearly zero, flat dispersion (Optics Letters, Vol. 25, No. 11, pp. 790-792, Jun. 1, 2000). Fibres with such dispersion properties are of large interest as non-linear fibres, since low dispersion (close to zero) is one of the key elements in order to reduce the threshold for non-linear effects. A further key element is to reduce the mode field diameter of the guide mode as much as possible. Hence, small cores and near-zero dispersion are desired characteristics for non-linear fibres. In order to utilize non-linear effects in fibres over broader wavelength ranges (such as in modem broadband telecommunication systems), it is further desired that the near-zero dispersion is obtained over a broad wavelength range in the near-infrared (hence, that the dispersion curve is relatively flat over the wavelength range of interest). It is an advantage of the fibres disclosed by Ferrando et al. that they exhibit flat, near-zero dispersion over a very broad wavelength range (for example dispersion between +1 ps/nm/km and xe2x88x921 ps/nm/km over more than 50 nm).
It is, however, a disadvantage of the fibres with near-zero, flat dispersion disclosed by Ferrando et al. that the centre-to-centre, xcex9, spacing between two nearest air holes surrounding the core is equal to or larger than 2.3 xcexcmxe2x80x94thereby limiting the smallest possible core diameter to around 4.6 xcexcm. The exact, absolute value of the core diameter, naturally, depends on the definition of the core diameter. Unless otherwise stated, we will use the same core diameter definition as used in WO 99/00685, namely a core diameter defined as the distance from a centre of an innermost cladding feature to a centre of another innermost cladding feature, these two innermost cladding features being positioned substantially opposite each other with respect to the core center. For the fibre design used by Ferrando et al. this results in a core diametre equal to two times xcex9 (hence Ferrando et al. disclose fibres with core diameters larger than 4.6 xcexcm). Alternatively, the core diameter may be defined from a circle connecting centers of the innermost cladding featuresxe2x80x94this may be relevant in the case of only a few innermost cladding features (such as 3 or 5). This later definition is in agreement with the core definition used in WO 99/00685. For core shapes with a strong deviation away from a circular shape, such as an elliptical or rectangular shape the core diameter should be defined most appropriate with respect to the mode field area, e.g. as the mid-value between lengths of the first and second main axes of the elliptical or rectangular shape.
It is an object of the present invention to provide optical fibers for non-linear applications, where the fibres are characterized by a core diameter smaller than 4 xcexcm as well as near-zero dispersion at near-infrared wavelength. In particular, it is an object to provide optical fibres with a core diameter smaller than 3.5 xcexcm and dispersion that varies less than +/xe2x88x925 ps/nm/km around zero over a wavelength range from at least 1.45 xcexcm to 1.65 xcexcm.
Glossary and Definitions
In this application we distinguish between xe2x80x9crefractive indexxe2x80x9d and xe2x80x9ceffective refractive indexxe2x80x9d. The refractive index is the conventional refractive index of a homogeneous material. In this application we consider mainly optical wavelengths in the visible to near-infrared regime (wavelengths from approximately 400 nm to 2 xcexcm). In this wavelength range most relevant materials for fibre production (e.g. silica) may be considered mainly wavelength independent, or at least not strongly wavelength dependent. However, for non-homogeneous materials, such as micro-structures, the effective refractive index is very dependent on the morphology of the material. Furthermore, the effective refractive index of a micro-structure is strongly wavelength dependentxe2x80x94much stronger than the refractive index of any of the materials composing the micro-structure. The procedure of determining the effective refractive index of a given micro-structure at a given wavelength is well-known to those skilled in the art (see e.g. Jouannopoulos et al, xe2x80x9cPhotonic Crystalsxe2x80x9d, Princeton University Press, 1995 or Broeng et al, Optical Fiber Technology, Vol. 5, pp. 305-330, 1999). The present invention takes advantage of specific micro-structure morphologies and their strong wavelength dependency in a novel manner and discloses fibres where the effective indices of the core and cladding regions are varying with respect to each-other in an untraditional way. Most importantly, there exists for certain fibres, disclosed in this application, specific wavelengthsxe2x80x94so-called xe2x80x9cshiftingxe2x80x9d wavelengthsxe2x80x94for which the difference between the effective indices of core and cladding regions may change sign. The present inventors utilize this property to realize micro-structured fibres with strong dispersion around the shifting wavelengths.
Usually a numerical method capable of solving Maxwell""s equation on full vectorial form is required for accurate determination of the effective refractive indices of micro-structures. The present invention makes use of employing such a method that has been well-documented in the literature (see previous Joannopoulos-reference). In the long-wavelength regime, the effective refractive index is roughly identical to the weighted average of the refractive indices of the constituents of the material. For micro-structures, a directly measurable quantity is the so-called filling fraction that is the volume of disposed features in a micro-structure relative to the total volume of a micro-structure. Of course, for fibres that are invariant in the axial fibre direction, the filling fraction may be determined from direct inspection of the fibre cross-section.
A problem to be solved by the invention is to be able to guide light in single-mode micro-structured fibres with relatively large mode areas, where either the zero dispersion wavelength is shorter than 1.3 xcexcm or the fibres exhibit a large negative dispersion value around 1.55 xcexcm. Another problem to be solved is to provide in single-mode micro-structured fibres with very small mode areas and a flat, near-zero dispersion at wavelengths around 1.55 xcexcm.
The present invention covers several aspects of fibres with special dispersion properties, namely fibres with a micro-structured core region and a micro-structured cladding region with cladding features being large compared to the wavelength of light guided through the fibres, and further, the invention covers fibres with a so-called raised, inner, micro-structured cladding region.
In certain aspects, the present invention relates to single-mode optical fibres with strong dispersion. A potential application of such fibres is for dispersion compensation or dispersion slope compensation of high bit rate optical communication links. For such applications, non-linear effects are crucial to eliminate, and fibres with relatively large mode field diameters are required.
The present inventors have realised that the prior art fibres with a solid core require small core diameters in order to obtain single-mode operation and large negative dispersion. For modern optical telecommunication systems based on standard transmission fibres with a zero-dispersion wavelength of 1.3 xcexcm, a very important fibre application today is dispersion compensation at wavelengths around at 1.55 xcexcm. However, for high capacity systems that are based on multi-wavelength channels (so-called dense wavelength multiplexed systems D-WDM), the prior art micro-structured fibres cannot be used for dispersion compensation due to their small core diameter. The small core diameter causes a (for this aspect) crucial increase in undesired non-linear effectsxe2x80x94such as e.g. four-wave mixing.
The present invention, however, discloses a number of fibre designs allowing realization of very large negative dispersion for large mode area fibres. In particular, for fibre core diameters which are comparable to those of standard optical transmission fibres.
The present inventors have further realized that the use of a depressed claddingxe2x80x94as disclosed in the prior artxe2x80x94is not optimum for realising fibres with large negative dispersion at near-infrared wavelengths. The present inventors have, on the other hand, found that micro-structured fibres can be improved with respect to increasing the dispersion (both to large negative or large positive values) if the fibres have a micro-structured core region and a micro-structured cladding region with cladding features being large compared to the wavelength of light guided through the fibre and/or if the fibre is designed with two cladding regions where the inner cladding region is micro-structured and has an effective refractive index that is larger than the outer cladding region at the operating wavelengths (the inner cladding region should e.g. have a lower filling fraction than the outer cladding region). We shall refer to this type of design as micro-structured fibres with a raised, inner cladding.
The present inventors have further realized that the use of a raised, inner cladding provides the flexibility to obtain fibres with very, small cores and near-zero dispersion over a broad wavelengths range at near-infrared wavelengths. Such fibres are covered by another aspect of the present invention, and are mainly of interest for non-linear applications.
Hence, the present inventors have realized that utilization of a raised, inner cladding is a particularly attractive for manipulating the dispersion properties of micro-structured fibres.
When tailoring the dispersion properties of micro-structured fibres, it is generally desired to have larger features the cladding (usually low-index features in the form of voids or air holes) as this provides larger effective, refractive index contrasts and thereby stronger dispersive effects. It is, therefore, desirable to realize micro-structured fibres with large air holes. The main problem for the fibre designs disclosed in the prior art is, however, that above a certain air hole size, the fibres may become multi-mode. The present invention describes a way of realizing strictly single-mode micro-structured fibres with large air holes, by using micro-structuring of the core region and/or various dopants in the high-index composite of the micro-structured fibre (this being either in the core or in the claddingxe2x80x94or in both).
According to a first aspect of the invention there is provided a micro-structured optical fibre for transmitting at least a predetermined wavelength of light, said optical fibre having an axial direction and a cross section perpendicular to said direction. The fibre comprises a core region that comprises a multiplicity of spaced apart core features that are elongated in the fibre axial direction and disposed in a core material. The presence of the core features cause a micro-structuring of the core region, and the effective refractive index of the core region is noted Nco. Surrounding the core region is a cladding material that comprises a multiplicity of spaced apart cladding features. These cladding features are elongated in the fibre axial direction and disposed in a first cladding material. The presence of the cladding features cause a micro-structuring of the cladding region, and the effective index of the cladding is noted Ncl. The fibre has a plurality of the cladding features that are large compared to the predetermined wavelength of the light guided through the fibre.
The micro-structured fibre according to the first aspect of the invention may be embedded in an article, which e.g. can be used in an optical fibre communication system.
By having a plurality of the cladding features with a large size compared to the wavelength of light guided through the fibre (the features are in at least one cross-sectional direction larger than the predetermined wavelength) and the micro-structured core region, it is possible to obtain the technical advantage of having a fibre with both strongly dispersive waveguiding properties and a large mode area. It is a requirement in order to obtain these characteristics that a plurality of the cladding features are large compared to the predetermined wavelength of the light guided through the fibre, while at the same time the core is micro-structured.
According to an embodiment of the first aspect of the invention the effective index of refraction of the core region, Nco, may be larger than the effective index of refraction of the cladding region, Ncl, at said predetermined wavelength of light. This is in order to ensure that the fibre may guide light in a single-mode by internal reflection at the predetermined wavelength.
It should be understood that the first aspect of the invention may cover a relatively large number of combinations or variations of the refractive indices of the core material, the core features, the cladding material, and the cladding features. Thus, the refractive index of one or more of the core features may be lower than the refractive index of the core material. Here, the refractive index of a majority or all of the core features may be lower than the refractive index of the core material. However, it is also within an embodiment of the first aspect of the invention that the refractive index of one or more of the core features is higher than the refractive index of the core material, where the refractive index of a majority or all of the core features may be higher than the refractive index of the core material.
Accordingly, the invention covers embodiments where the refractive index of one or more of the cladding features is lower than the refractive index of the cladding material, where the refractive index of a majority or all of the cladding features may be lower than the refractive index of the cladding material. However, the invention further covers embodiments, where the refractive index of one or more of the cladding features is higher than the refractive index of the cladding material, and the refractive index of a majority or all of the cladding features may be higher than the refractive index of the cladding material.
The fibres disclosed in the present invention are intended for use in a wide range of applications, where the light guided trough the fibre may be in the range from 0.3 xcexcm to 2 xcexcm. For use in certain systems, the predetermined wavelength may be very shortxe2x80x94typically in the interval from 0.3 xcexcm to 0.6 xcexcm. For other applications, the fibre may be desired for delivery of light from laser sources such as III-V semiconductor lasersxe2x80x94with a wavelength range from around 0.6 xcexcm to 1.2 xcexcm. Particularly, the wavelength range around 0.8 xcexcm is of interest for delivery of light from relatively cheap GaAs based semiconductor lasers. For other applications, fibres according to the present invention may be used for applications such as delivery of light from powerful, tuneable Ti:Sapphire lasers. Hence, the fibres may be designed to guide light at wavelengths between 0.78 xcexcm to 0.98 xcexcm. For other systems, e.g. systems employing lasers and amplifiers based on rare-earth doping, the fibres may be desired to guide light at specific wavelengths, corresponding to transitions for particular rare-earths. Important transition lines are located around 1.06 xcexcm and 1.55 xcexcm. The fibres according to the present invention, may be used for a number of telecommunication applicationsxe2x80x94e.g for dispersion compensationxe2x80x94where the fibre may be used in the wavelength range from about 1.2 xcexcm to 1.6 xcexcm. Particularly, the fibres may find use in the so-called second and third telecommunication window, i.e. for wavelengths around 1.3 xcexcm and for wavelengths from around 1.5 xcexcm to 1.6 xcexcm. For yet other applications, the fibres may find use at mid-infrared wavelengths, such as around 2.0 xcexcm. The present invention covers preferred embodiments, where the predetermined wavelength is within the above-mentioned wavelength ranges.
In a preferred embodiment, fibres according to the present invention have single-mode operation. This property is important both for applications at short wavelengthsxe2x80x94e.g. for lithographic applicationsxe2x80x94as well as for longer wavelength applications, such as telecommunication applications around 1.55 xcexcm.
Often, a fibre according to the present invention should also guide light for a range of wavelengths below the predetermined wavelength. This is e.g. the case for wavelength multiplexing systems used in telecommunication systems, where the fibre should be single mode at wavelength in the range from 1.5 xcexcm to 1.6 xcexcm. For other applications, such as fibre amplifiers or fibre laser, the fibres are desired to be single-mode at a pump wavelength that may be significantly below the predetermined wavelength. Therefore, preferred embodiments of the present invention covers fibres with single mode operation for wavelength ranges down to 0.3 xcexcm.
It is for many applications desirable to have fibres according to the present invention to operate by total internal reflection. In a preferred embodiment, the core features are, therefore, smaller in size compared to the cladding features. This may provide a high-index core region within which light can be guided by total internal reflection (this type of guidance is in the literature also referred to as index-guidance or modified total internal reflection). The size is in this respect easily determined from an inspection of the fibre cross-sectionxe2x80x94the core features should have at least one cross-sectional dimension that is smaller than that of the cladding features. It is preferred that a part of or all of the core features have cross-sectional dimensions perpendicular to said axial direction being smaller than the cross-sectional dimensions of the cladding features.
The present invention includes micro-structured fibres, where the elongated features may be either non-periodically or periodically distributed. Hence, when we are discussing the spacing of elongated elements, we refer to the centre-to-centre distance between two neighbouring features. For periodically distributed features, this centre-to-centre spacing is easily determined, and is e.g. for a close-packed arrangement of the features identical to the pitch of the periodic structure. For non-periodic distributions, the centre-to-centre spacing should be taken as the average centre-to-centre distance between neighbouring features in the relevant region. For special distributions, e.g. in the case of a very low number of features, the centre-to-centre spacing should be taken as the smallest centre-to-centre distance between neighbouring features in the relevant region.
In a preferred embodiment, the core features have a centre-to-centre spacing that is smaller than the predetermined wavelength of light guided through the fibre. This provides a further improvement in order to obtain operation by total internal reflection. In further preferred embodiments, the core feature spacing is smaller than 0.9 times the predetermined wavelength, such as smaller than 0.6 times, such as smaller than 0.4 times, or such small as 0.2 times the predetermined wavelength.
To further ensure an operation based on total internal reflection, it is further preferred to have the core features small compared to the predetermined wavelength. Hence, in a preferred embodiment the core features have a cross-sectional dimension that is smaller than the predetermined wavelength. Thus, a part of or all of the core features may have cross-sectional dimensions perpendicular to said axial direction being smaller than 0.9 times the predetermined wavelength, such as 0.6 times, such as smaller than 0.4 times, or such as smaller than 0.2 times. Preferably, the cross-sectional dimension is as small as 0.2 times the predetermined wavelength.
Since fibres according to the first aspect of the present invention are characterized by relatively large cladding features (a plurality being larger than the predetermined wavelength), the core features may need to have a certain size in order to ensure single mode operation at the predetermined wavelength. In a preferred embodiment the core features have a cross-sectional dimension that is larger than 0.2 xcexcm.
It is preferred that the core features have a cross-sectional dimension perpendicular to said axial direction being so large that a second-order mode of propagation is shifted to a wavelength of light being shorter or smaller than said predetermined wavelength. In a preferred embodiment, the core features are so large that the second-order mode will only be able to propagate at wavelength shorter than 1.5 xcexcm, such as smaller than 1.3 xcexcm, or such as smaller than 1.06 xcexcm, such as smaller than 0.8 xcexcm, or such as smaller than 0.6 xcexcm, such as smaller than 0.4 xcexcm, such as smaller than 0.3 xcexcm, or such as smaller than 0.2 xcexcm. Preferably, the second-order mode cut off is shifted to wavelengths as short as 0.2 xcexcm. For single-mode fibres, it should be mentioned that the second-order mode cut-off is often simply referred to as the cut-off.
To obtain the above-discussed shifting of the second-order mode cut-off, it is preferred that the core features have a cross-sectional dimension smaller than 2 xcexcm, such as smaller than 1.3 xcexcm, such as smaller than 1.06 xcexcm, such as smaller than 0.8 xcexcm, or such as smaller than 0.6 xcexcm, such as smaller than 0.4 xcexcm, such as smaller than 0.3 xcexcm, or such as smaller than 0.2 xcexcm. In a preferred embodiment, the core features have a cross-sectional dimension in the range from 0.2 xcexcm to 1.8 xcexcm.
In has already been mentioned that in order to ensure that the fibres according to the present invention are single-mode at the predetermined wavelength when operating by total or almost total internal reflection, the effective refractive index of the core region should be larger than the effective refractive index of the cladding region. Hence, in a preferred embodiment, Nco is larger than Ncl at the predetermined wavelength.
In order to obtain a strongly dispersive waveguide characteristic, the present inventors have realized how to utilize that the cladding region can be designed to have a higher effective refractive index than the core at wavelengths shorter than the predetermined wavelength (where Nco is larger than Ncl). This provides a cut-off for the fundamental mode in the core region at a so-called shifting wavelength (where Nco and Ncl are equal), but a very strong dispersion at the (longer) predetermined wavelength. Therefore, the present invention covers fibres where Ncl is larger than Nco below a shifting wavelength, this shifting wavelength being shorter than the predetermined wavelength. The present inventors have realized that the dispersion is strongest close to the shifting wavelength, and depending on the application of the fibres, the shifting wavelength may, therefore, be tailored to a specific value. In order to obtain the strong dispersion at particular predetermined wavelengths, the present invention, therefore, covers preferred embodiments, where the shifting wavelength is below 1.5 xcexcm, where it is below 1.3 xcexcm, below 1.06 xcexcm, below 0.8 xcexcm, below 0.6 xcexcm, and below 0.4 xcexcm.
Yet another manner of ensuring operation by total internal reflection, is by having the feature filling fraction of the core region lower than the cladding region. Hence, in a preferred embodiment, the core features in the cross-section occupy in total a ratio Fc of the core region that is smaller than the ratio Fi, where Fi is the total ratio that the cladding features occupy of the cladding region.
The present invention also covers fibres that may guide light by PBG (Photonic Band Gap) effects. In this case, the cladding features must be periodically distributed, and the present invention therefore includes preferred embodiments where the cladding features are periodical features. The cladding features may e.g. be arranged in a close-packed arrangement, which provides intrinsically the largest feature filling fraction. A long range of other arrangements may, however, also be of interest for specific applications.
To provide fibres according to the present invention that are operating solely by PBG effects it is preferred that the core region has a lower effective refractive index than the cladding region. Thus, in order to operate in the PBG mode, the effective index of refraction of the core region, Nco, should be lower than the effective index of refraction of the cladding region, Ncl, at said predetermined wavelength of light.
Hence, in a preferred embodiment, the core features may have cross-sectional dimensions perpendicular to said axial direction being larger than the cross-sectional dimensions of the cladding features. The larger core features or holes may provide a low-index core regionxe2x80x94meaning that the effective refractive index of the core region may be lower than the effective refractive index of the cladding regionxe2x80x94within which light may be guided by PBG effect for certain wavelength ranges. For a specific fibre, these wavelength ranges may be tailored by choice of cladding feature arrangement, adjustment of cladding features size(s), core feature arrangement and/or core feature size(s). Such fibres may be advantageous not only in order to realize fibre with the previously described dispersive properties, but additionally to be able to guide a high fraction of light in the core features. In the case of air or vacuum filled features such fibres may, therefore, further be characterized by low material losses.
Another manner of ensuring operation solely by PBG effects is by having the feature filling fraction of the core region higher than the cladding region. Hence, in a preferred embodiment, the core features in the cross-section occupy in total a ratio Fc of the core region that is larger than the ratio Fi, where Fi is the total ratio that the cladding features occupy of the cladding region.
In order to fabricate micro-structured fibres with a highly regular feature arrangement, it is preferred to have the centre-to-centre spacing between core features equal to the centre-to-centre spacing of cladding features. Hence, the present invention covers preferred embodiments with a substantially identical feature spacing relation for the core and cladding regions. Due to structural uniformities during fabrication, the spacing may, however, in practice have smaller variations, even in the case where an identical spacing is sought.
In yet another preferred embodiment, the core feature spacing is smaller than the cladding feature spacing. The main advantage of this is the achievement of a higher flexibility when tailoring the mode-shape of light guided through the fibre. By using core features that are smaller than the cladding features, it is possible to increase the number of core features and thereby to provide a better mode shaping. For most applications, it is desired to have a mode-shape that is as close as possible to a gaussian shape, in order to reduce coupling losses at e.g. splicing to standard fibres.
To provide the largest flexibility for mode-shaping, it is preferred that the number of core features is larger than two. To further increase the mode-shaping flexibility, it is preferred that the number of core features is larger than 5, and even further preferable that the number of core features is larger than 17.
The fabrication method most commonly used for the fabrication of micro-structured fibre, would for the realization of fibres according to the present invention favour the use of specific numbers of core features (when these have a size smaller than the cladding featuresxe2x80x94preferably a cladding feature spacing of three times or of the square-root of three times the core feature spacing). Therefore, in a preferred embodiment, the number of core features is equal to 7, or is equal to 13 or is equal to 19.
In yet another preferred embodiment, the core material (the background material of the core region) has a lower refractive index than the cladding material. This allows a further flexibility of tailoring the dispersive properties of fibres guiding by either of the two types of waveguidance (total internal reflection, PBG effects or both). For example, it is possible to provide further control of the above-mentioned shifting wavelength by the use of a core material having a lower refractive index compared to the cladding material. This refractive index difference may be obtained e.g. by using different dopants in the two materials (e.g. silica doped to various degrees), or it may be obtained simply by using different basis materials (e.g. different types of polymers).
In yet another preferred embodiment the refractive index of the core and cladding materials are identical or substantially equal. This may e.g. be preferred in cases where fibre losses are a critical issue, and the fibre must be fabricated from the purest possible material. In this case it is preferred to use the same (pure) material for the core and cladding material. Also with respect to fabrication method, it may be an advantage to use the same core and cladding material (and therefore the same refractive index of the core and cladding material). This is e.g. the case where a difference in thermal expansion coefficient for the core and cladding materials cannot be tolerated. The presently used fabrication methods for micro-structured fibres are generally not in favour of the use of different core and cladding materials. Hence, fibres with the same core and cladding material are preferred.
It should, however, be understood that the present invention also covers embodiments in which the refractive index of the core material is higher than the refractive index of the cladding material.
In order to obtain the strongest possible dispersive properties of the fibres according to the present invention, it is preferred that a high fraction of the cladding features have a cross-sectional dimension that is larger than the predetermined wavelength. Hence, preferred embodiments covers fibres, where more than 20% of the cladding features are larger than the predetermined wavelength, such as more than 40%, more than 60%, or more than 80% or all of the cladding features are larger.
In a preferred embodiment, the core has a diameter larger than 2 xcexcm. Generally, for telecom applications a core size in the range from about 2 xcexcm to 10 xcexcm is desired. For high-power applications, a larger core size is desired such as from about 10 xcexcm to 50 xcexcm.
In order to obtain the strongest possible dispersive effects in fibres according to the present invention, it is preferred to have the cladding feature size as large as possible. Hence, in a further preferred embodiment, the cladding features should have a diameter that is larger than 0.45 times the cladding feature spacing, such as a diameter larger than 0.6 times the cladding feature spacing, such as larger than 0.9 times the cladding feature spacing. Also it is preferred that cladding features occupy at least 25% of the cross-section of the cladding region, such as more than 40%, such as more than 50%, such as more than 60%, such as more than 70%, such as more than 80%.
It is further preferred that in order to guide light in a single mode with strong dispersion, that the core features occupy more than 5% of the cross-section of the core region, such as more than 10%, such as more than 25%, such as more than 50%, such as more than 75%.
In a further preferred embodiment the core features are periodical core features. This allows the simplest manner for fabrication of fibres with a micro-structured core region.
In a further preferred embodiment the spacing of the core features and of the cladding features are in the range of about 0.2 xcexcm to 10 xcexcm.
Commonly it is preferred to realise the fibres with core material and/or the cladding material being silica.
The core features and/or the cladding features may be rods or voids or combinations of rods and voids. Thus, one or more, a majority or all of the core features may be rods. It is also within embodiments of the invention that one or more, a majority or all of the cladding features are rods. Here, some or all of the rods of the core features and/or the cladding features may be made of silica, wherein one or more of the silica core features and/or cladding features may be doped with one or more materials selected from a list comprising: Ge, Al, F, Er, Yb, Nd, La. However, the invention also covers embodiments wherein one or more, a majority or all of the core features are voids. Similarly, one or more, a majority or all of the cladding features may be voids.
When some of the core features and/or the cladding features are voids, these voids may, depending on the specific application of the fibre, contain air, another gas, or a vacuum. Alternatively, when any of the core features and/or the cladding features are voids, such voids may contain polymer(s), a material providing an increased third-order non-linearity, a photo-sensitive material, or a rare earth material. In a second aspect of the invention there is provided an article (e.g. an optical fibre communication system) that comprises a micro-structured optical fibre for guiding light at an operating wavelength, said optical fibre having an axial direction and a cross section perpendicular to said axial direction, the optical fibre comprising: a core region having an effective refractive index Nco and being surrounded by a cladding region comprising a multiplicity of spaced apart cladding features being elongated in the axial direction and disposed in a first cladding material, the cladding features having a refractive index that differs from a refractive index of the first cladding material, the cladding region further comprising an inner cladding region surrounding the core region and an outer cladding region surrounding the inner cladding region where the inner and outer cladding regions have effective refractive indices Ni, and No, respectively, with Ni greater than No at the operating wavelength.
It should be understood that the article of the second aspect of the invention may be an optical fibre communicating system or a part of an optical fibre communicating system such as the micro-structured optical fibre itself.
In an embodiment of the second aspect of the invention, the inner cladding region comprises the first cladding material and the cladding features disposed therein, the first cladding material thereby constituting an inner cladding material and the cladding features disposed in the first cladding material constituting a multiplicity of spaced apart inner cladding features. It is further preferred that the outer cladding region comprises a multiplicity of spaced apart outer cladding features being elongated in the axial direction and disposed in an outer cladding material, the outer cladding features having a refractive index that differs from a refractive index of the outer cladding material.
In a preferred embodiment of the second aspect of the invention there is provided an article comprising a micro-structured optical fibre for guiding light at an operating wavelength, said optical fibre having an axial direction and a cross section perpendicular to said axial direction, the optical fibre comprising a core region surrounded by an inner cladding region that comprises a multiplicity of spaced apart inner cladding features that are elongated in the axial direction and disposed in an inner cladding material, the inner cladding region being surrounded by an outer cladding region that comprises a multiplicity of spaced apart outer cladding features that are elongated in the axial direction and disposed in an outer material, the inner cladding features having a refractive index that differs from a refractive index of the inner cladding material and the inner cladding region having an effective refractive index Ni, and the outer cladding features having a refractive index that differs from a refractive index of the outer cladding material and the outer cladding region having an effective refractive index No, wherein Ni is larger than No at the operating wavelength.
It has already been mentioned that for a micro-structure fibre, such as a fibre having a cladding region with cladding features, the effective refractive index may be strongly wavelength dependent. Thus, for embodiments of the second aspect of the invention having cladding features in the cladding regions, and wherein the core region has an effective refractive index Nco, it is preferred that optical fibre is dimensioned so that the difference between Nco and Ni is a function of the wavelength of the guided light, and so that the effective index of the core region Nco is substantially equal to the effective index of the inner cladding region Ni at a wavelength referred to as the shifting wavelength, and wherein Ni is larger than No for operating wavelengths equal to or below said shifting wavelength.
In one embodiment, the optical fibre may be dimensioned so that Nco is larger than Ni for wavelengths or at least a range of wavelengths longer than the shifting wavelength, and Nco is lower than Ni for wavelengths or at least a range of wavelengths shorter than the shifting wavelength. In another embodiment, the optical fibre may be dimensioned so that Nco is lower than Ni for wavelengths or at least a range of wavelengths longer than the shifting wavelength, and Nco is higher than Ni for wavelengths or at least a range of wavelengths shorter than the shifting wavelength. The second aspect of the invention also covers embodiments wherein Ni is larger than No for a range of wavelengths above the shifting wavelengths.
It should be understood that the second aspect of invention covers embodiments wherein the core region is a substantially solid core made of a core material and having an effective refractive index Nco being substantially equal to the refractive index of the core material. However, the second aspect of invention also covers embodiments, wherein the core region comprises a multitude of spaced apart core features being elongated in the axial direction and disposed in a core material.
In an embodiment of the second aspect of the invention the inner cladding features or at least part of the inner cladding features may have a cross-sectional dimension that is smaller than a cross-sectional dimension of the outer cladding features. This provides a relatively easy design to allow realisation of raised cladding micro-structured fibres. Typically the features are substantially circular, thus the cross-sectional dimension may be equal to the feature diameter.
In a preferred embodiment, the refractive index of the core material is lower than the refractive index of the inner cladding region material. This allows a simple design for a fibre exhibiting a shifting wavelength.
It is also within a preferred embodiment that the centre-to-centre spacing between inner and outer cladding features is substantially identical in the meaning that the centre-to-centre spacing between two nearest inner cladding features is substantially identical to the centre-to-centre spacing between two nearest outer cladding features. The advantages of this embodiment are similar to those described in the first aspect of the invention.
According to an embodiment of the second aspect of the invention, the filling fraction of inner cladding features in the inner cladding region is smaller than the filling fraction of outer cladding features in the outer cladding region.
Thus, the inner cladding features may in the cross-section occupy in total a ratio, Fi, of the inner cladding region and the outer cladding features may in the cross-section occupy in total a ration, Fo, of the outer cladding region, with Fi being smaller than Fo. This provides a simple manner of obtaining a raised inner cladding.
It should be understood that the characteristics of a micro-structured fibre according to the second aspect of the invention may be obtained by a relatively large number of different fibre designs. Thus, the second aspect of the invention may also cover embodiments, wherein all or at least part of the inner cladding features have a cross-sectional dimension being substantially identical to a cross-sectional dimension of all or at least part of the outer cladding features. Here, it is preferred that the centre-to-centre spacing between inner cladding features is larger than the centre-to-centre spacing between outer cladding features.
According to an embodiment of the invention, the number of inner cladding features may be lower than 6, such as equal to 4, such as equal to 3, such as equal to 2.
It is also within embodiments of the second aspect of the invention that the refractive index of the inner cladding material is substantially identical to or larger than the refractive index of the outer cladding material.
In a preferred embodiment, the refractive index of the inner cladding material is larger than the refractive index of the outer cladding material and the inner cladding features in the cross-section occupy an area, Fi, of the inner cladding region and the outer cladding features in the cross-section occupy an area, Fo, of the outer cladding region, and Fi is equal to or larger than Fo. In this preferred embodiment, the higher refractive index of the inner cladding material allows the use of larger inner cladding features, while the fibre maintains the relation Ni greater than No.
According to an embodiment of the invention, the refractive index of one or more of the inner cladding features may be higher than the refractive index of the inner cladding material. This may also help in obtaining a raised inner cladding region. Here, the refractive index of a majority or all of the inner cladding features may be higher than the refractive index of the inner cladding material.
However, it is also within embodiments of the invention that the refractive index of one or more of the inner cladding features is lower than the refractive index of the inner cladding material. Here, the refractive index of a majority or all of the inner cladding features may be lower than the refractive index of the inner cladding material.
The second aspect of the present invention also covers an embodiment in which the outer cladding region comprises the first cladding material and the cladding features disposed therein, the first cladding material thereby constituting an outer cladding material and the cladding features disposed in the first cladding material constituting a multiplicity of spaced apart outer cladding features. Here, the core region may comprise a multitude of spaced apart core features being elongated in the axial direction and disposed in a core material. In one embodiment the refractive index of the core features is larger than the refractive index of the inner cladding material, and the refractive index of the core material is lower than the refractive index of the inner cladding material. However, in an alternative embodiment, the refractive index of the core features may be lower than the refractive index of the inner cladding material, and the refractive index of the core material may be larger than the refractive index of the inner cladding material. According to an embodiment, the inner cladding region may here comprise an inner cladding material with the refractive index of said inner cladding material being larger than the refractive index of the outer cladding features. Preferably, the refractive index of the inner cladding material may be about equal to or larger than the refractive index of the outer cladding material. For fibres according to the second aspect of the invention having an outer cladding region with outer cladding features, the refractive index of one or more, a majority or all of the outer cladding features may be higher than the refractive index of the outer cladding material. However, in a preferred embodiment, the refractive index of one or more, a majority or all of the outer cladding features is lower than the refractive index of the outer cladding material.
For fibres according to the second aspect of the invention having a core region with core features, the refractive index of one or more of the core features may be higher than the refractive index of the core material. However, in a preferred embodiment the refractive index of one or more of the core features is lower than the refractive index of the core material.
In an embodiment of the second aspect of the invention, the refractive index of the core material is substantially identical to the refractive index of the inner cladding region material. The advantages of this embodiment are similar to those described in the first aspect of the invention. In another or further embodiment, the refractive index of the core material is substantially identical to the refractive index of the outer cladding region material. The advantages of this embodiment are similar to those described in the first aspect of the invention.
For embodiments according to the second aspect having core features, the core features may have a diameter that is smaller than the diameter of the inner cladding features. This may ensure operation by total internal reflection. It is also within an embodiment of the invention that the core features have a centre-to-centre spacing being smaller than the centre-to-centre spacing of the inner cladding features. This allows further means for mode shaping.
For fibres according to the second aspect of the invention having outer cladding features, the outer cladding features may occupy more than 30% of the cross-section of the outer cladding region, such as more than 40%, such as more than 50%, such as more than 60%, such as more than 70%, such as more than 80%. This allows stronger dispersion to be exhibited.
In a preferred embodiment of the second aspect of the invention, the inner and/or outer cladding features may be periodically disposed. This may allow operation by PBG effects, as well as it may be of importance for reasons of fabrication, as the one of the most widely used fabrication techniques favours the use of close-packed cladding featuresxe2x80x94hence, the features may be periodic features. It is also within an embodiment of the invention that the core features are periodical core features. This is advantageous for similar reasons as described in the first aspect of the invention.
In a preferred embodiment of the second aspect of the invention, the core has a diameter larger than 2 xcexcm. A relatively large mode area is vital for the use of fibres in e.g. multi-wavelength dispersion compensation schemes. The advantages of this preferred embodiment are similar to those described in the first aspect of the invention. Here, the core diameter may be in the interval from 2 xcexcm to 10 xcexcm, such as in the interval from 4 xcexcm to 6 xcexcm. The advantages of having core diameters within these ranges are to provide fibres for high power applications, for lowering/elimination of non-linear effects, and/or for low coupling losses to standard transmission fibres.
For fibres according to the second aspect of the invention, the inner and/or outer cladding features may have a spacing in the range of about 0.1 to 10 times the wavelength of any light guided through the fibre, such as in the range of about 0.5 to 1, such as in the range of about 1 to 2, such as in the range of about 2 to 5, such as in the range of about 5 to 10. The potential of dimensioning the cladding features within the above-described ranges provides a high flexibility when tailoring the dispersion properties for specific applications. It is important that strong dispersion may well be obtained for small cladding features. This is attributed to the fact that a raised inner cladding region may well be obtained even for small cladding featuresxe2x80x94hence, providing a shifting wavelength.
In a preferred embodiment of the second aspect of the invention, the core features may have a spacing in the range of about 0.1 to 10 times the wavelength of any light guided through the fibre, such as in the range of about 0.5 to 1, such as in the range of about 1 to 2, such as in the range of about 2 to 5, such as in the range of about 5 to 10. As for the above-described embodiment, this embodiment ensures a large flexibility of the present invention with respect to specific applications. The second aspect of the present invention also covers embodiments where the core features have a spacing in the range of about 0.1 xcexcm to 10 xcexcm, such as in the range of about 0.5 xcexcm to 1 xcexcm, such as in the range of about 1 xcexcm to 2 xcexcm, such as in the range of about 2 xcexcm to 5 xcexcm, such as in the range of about 5 xcexcm to 10 xcexcm. Again this embodiment ensures a large flexibility of the present invention with respect to specific applications.
For the fibres according to the second aspect of the invention, any of the core features and/or any of the inner or the outer cladding features may be voids. The advantages of this preferred embodiment are similar to those described in the first aspect of the invention. Thus, one or more, a majority or all of the core features may be voids and/or one or more, a majority or all of the inner cladding features may be voids. It is also within the second aspect of the invention that one or more, a majority or all of the outer cladding features are voids. Here, one or more of the core features and/or the cladding features may be voids containing air, another gas, or a vacuum.
However, it is also within the second aspect of the fibres that any of the core features and/or any of the inner or the outer cladding features may be rods. Thus, one or more, a majority or all of the core features may be rods and/or one or more, a majority or all of the inner cladding features may be rods. It is also within the second aspect of the invention that one or more, a majority or all of the outer cladding features may be rods.
Similar to embodiments of the first aspect of the invention, the second aspect of the invention also covers embodiments where one or more of the core features and/or the cladding features are voids containing polymer(s), a material providing an increased third-order non-linearity, a photo-sensitive material, or a rare earth material. Such embodiments allow the realization of various applications, such as fibre laser, amplifiers, wavelength converters, optical switches etc.
According to the present invention the fibres of both the first and the second aspect of the invention may be designed for guiding light with wavelength(s) in the range from about 0.3 xcexcm to 15 xcexcm, such as from about 0.5 xcexcm to 1.6 xcexcm, such as from about 1.0 xcexcm to 2.0 xcexcm, such as from about 2 xcexcm to 5 xcexcm, such as from about 5 xcexcm to 15 xcexcm. The fibres in this application will mainly be of interest for applications where light in various wavelength ranges between 0.3 xcexcm and 2 xcexcm, as discussed in the first aspect of this invention. Due to the fact that a significant fraction of the light may in fact be guided in the core featuresxe2x80x94and these may e.g. contain air or vacuumxe2x80x94the fibres presented in this application may well find use for guidance of light at wavelengths that are presently unattainable using conventional optical fibres. Hence, the fibres presented in this application may be utilized for guidance of light at both mid-infrared wavelengthsxe2x80x94the wavelength ranges from around 2 xcexcm to 5 xcexcmxe2x80x94as well as for mid- to far-infrared wavelengthsxe2x80x94such as the wavelength range from around 5 xcexcm to 15 xcexcm.
For a range of applications, such as e.g. dispersion compensation in D-WDM systems, it is preferred that the fibres according to the present invention are designed for guiding light at several predetermined wavelengths.
The present invention further relates to a third aspect, according to which there is provided an article comprising a micro-structured optical fibre for guiding light at an operating wavelength, said optical fibre having an axial direction and a cross section perpendicular to said axial direction, the optical fibre comprising: a core region having an effective refractive index Nco and being surrounded by an inner cladding region that comprises a multiplicity of spaced apart inner cladding features that are elongated in the axial direction and disposed in an inner cladding material, the inner cladding region being surrounded by an outer cladding region, the inner cladding features having a refractive index that differs from a refractive index of the inner cladding material, the inner cladding region having an effective refractive index Ni and the outer cladding region having an effective refractive index No, wherein Ni is larger than No at the operating wavelength, and wherein the core region is a substantially solid core made of a core material, said core region having an effective refractive index Nco being larger than Ni at the operating wavelength. It is preferred that the outer cladding region comprises a multiplicity of spaced apart outer cladding features being elongated in the axial direction and disposed in an outer cladding material, with the outer cladding features having a refractive index that differs from a refractive index of the outer cladding material.
It should be understood that the article according to any embodiment of the third aspect of the invention may be an optical fibre communicating system or a part of an optical fibre communicating system such as the micro-structured optical fibre itself.
In a preferred embodiment of the third aspect of the invention there is provided an article comprising a micro-structured optical fibre for guiding light at an operating wavelength, said optical fibre having an axial direction and a cross section perpendicular to said axial direction, the optical fibre comprising a core region having an effective refractive index Nco and being surrounded by an inner cladding region that comprises a multiplicity of spaced apart inner cladding features that are elongated in the axial direction and disposed in an inner cladding material, the inner cladding region being surrounded by an outer cladding region that comprises a multiplicity of spaced apart outer cladding features that are elongated in the axial direction and disposed in an outer material, the inner cladding features having a refractive index that differs from a refractive index of the inner cladding material and the inner cladding region having an effective refractive index Ni, and the outer cladding features having a refractive index that differs from a refractive index of the outer cladding material and the outer cladding region having an effective refractive index No, wherein:
Ni is larger than No at the operating wavelength, the core region is a substantially solid core with a core diameter around or below 4 xcexcm and with an effective refractive index Nco being larger than Ni at the operating wavelength, the centre to centre spacing or pitch of the inner cladding features, xcex9i, is around or below 2 xcexcm, the inner cladding features have a diameter or cross sectional dimension, di, fulfilling the requirement that di/xcex9i is equal to or below 0.7 and equal to or above 0.2, the centre to centre spacing or pitch of the outer cladding features, xcex9o, is around or below 2 xcexcm, and the outer cladding features have a diameter or cross sectional dimension, do, fulfilling the requirement that do/xcex9o is equal to or above 0.4. The micro-structured fibre exhibits non-linear optical effects at wavelengths around 1.5 xcexcm, such as in the wavelength range from 1.4 xcexcm to 1.6 xcexcm.
For the third aspect of the invention it is preferred that Nco is larger than Ni for all wavelengths of optical radiation which can be guided by the micro-structured fibre. It is also preferred that the effective refractive index difference between the core region and the inner cladding region is greater than about 5%.
According to a preferred embodiment of the third aspect of the invention, the core region and the inner cladding region are mutually adapted so that the micro-structured fibre exhibits a substantially zero dispersion or near-zero dispersion wavelength within the range of 1.2 xcexcm to 1.8 xcexcm, such as within the range of 1.48 xcexcm to 1.62 xcexcm, such as within the range of 1.52 xcexcm to 1.58 xcexcm. It is also within a preferred embodiment that the centre to centre spacing or pitch of the inner cladding features, xcex9i, is around or below 2 xcexcm, such as around or below 1.5 xcexcm, such as around 1.4 xcexcm, or in the range of 1 xcexcm to 2 xcexcm, such as in the range of 1 xcexcm to 1.5 xcexcm.
It is also in accordance with the third aspect of the invention that the number of inner cladding features is higher than or equal to 6. Here, the inner cladding features may have a diameter or cross sectional dimension, di, and a centre to centre spacing or pitch, xcex9i, fulfilling the requirement that di/xcex9i is in the range from 0.2 to 0.4, such as about 0.3. The third aspect also covers embodiments wherein the centre to centre spacing or pitch of the outer cladding features, xcex9o, is substantially equal to the centre to centre spacing or pitch of the inner cladding features, xcex9i.
In a preferred embodiment of the third aspect of the invention, all or at least part of the inner cladding features have a diameter or cross-sectional dimension being substantially identical to a diameter or cross-sectional dimension of all or at least part of the outer cladding features. The number of inner cladding features may be lower than 6, such as equal to 4, such as equal to 3, such as equal to 2, Here, it is preferred that the centre to centre spacing between inner cladding features xcex9i may be larger than the centre to centre spacing between outer cladding features xcex9o. Furthermore, the inner cladding features may have a diameter or cross sectional dimension, di, and a centre to centre spacing or pitch, xcex9i, fulfilling the requirement that di/xcex9i is in the range from 0.25 to 0.5, in the range from 0.28 to 0.57, or in the range from 0.35 to 0.7. It is further preferred that the outer cladding features are arranged in concentric manners forming concentric annular regions surrounding the core. Preferably the number of concentric rings of cladding features is two or larger than two, such as three, four or five or more rings. The separation between rings may be identical for all rings or the separation between rings may be varying in different manners in order to further tailor the waveguiding properties of the fibres. In a preferred embodiment, the separation between rings is decreasing. In another preferred embodiment, the separation between rings is increasing.
It is preferred that the centre to centre spacing or pitch of the outer cladding features, xcex9o, is around or below 2 xcexcm, such as around or below 1.5 xcexcm, such as around 1.4 xcexcm, or in the range of 1 xcexcm to 2 xcexcm, such as in the range of 1 xcexcm to 1.5 xcexcm. The outer cladding features may preferably have a diameter or cross sectional dimension, do, fulfilling the requirement that do/xcex9o is equal to or below 0.7 and equal to or above 0.4, such as about 0.5 or such as about 0.6.
For embodiments according to the third aspect of the invention, it is preferred that the core has a diameter around or below 4 xcexcm, such as around or below 3.6 xcexcm, such as around or below 2 xcexcm, such as around or below 1.5 xcexcm. It is also within an embodiment of the invention that the refractive index of the core region is varying along the diameter of the core region so that an inner and/or a central portion of the core has a higher refractive index than an outer portion of the core. The inner and/or central portion of the core may have a higher refractive index than the refractive index of the inner cladding material and/or the outer cladding material. In a preferred embodiment, the inner and/or central portion of the core may comprise one or more rods having a higher refractive index than the background index of the core or the outer portion of the core.
The third aspect of the invention also covers embodiments having different relations between the refractive indices of the core region and the inner and/or outer cladding regions. In one embodiment, at least part of the core region has a refractive index being substantially identical to the refractive index of the inner and/or outer cladding region material. In another embodiment, at least part of the core region has a refractive index being larger than the refractive index of the inner and/or outer cladding region material. In yet another embodiment, at least part of the core region has a lower refractive index than the refractive index of the inner cladding material and/or the outer cladding material
It is preferred that the inner cladding features are voids and/or rods having a lower refractive index than the inner cladding material. It is also preferred that the outer cladding features are voids and/or rods having a lower refractive index than the outer cladding material. For embodiments having cladding features being voids, such cladding features may be voids containing air, another gas, or a vacuum. The cladding feature may also or alternatively be voids containing polymer(s), a material providing an increased third-order non-linearity, a photo-sensitive material, or a rare earth material.
According to an embodiment of the third aspect of the invention, all or at least part of the inner cladding features may have a cross-sectional dimension being smaller than a cross-sectional dimension of the outer cladding features. It is also within an embodiment of the invention that the refractive index of the inner cladding material is substantially identical to the refractive index of the outer cladding material.
The third aspect of the invention also covers embodiments where the inner cladding features in the cross-section occupy in total a ratio, Fi, of the inner cladding region, and the outer cladding features in the cross-section occupy in total a ration, Fo, of the outer cladding region, and Fi is smaller than Fo.
It should be understood that the different materials may be selected as background material for the core region and/or the cladding regions for the fibres of the different aspects of the present invention. Thus, the core region and/or the cladding regions may comprise silica.
It also within embodiments of the aspects of the invention that the core and/or any of the cladding materials contains polymer(s), comprise a dopant (e.g. an active or photosensitive material) or a material showing higher order (non-linear) optical effects. Thus, the core region and/or any of the cladding regions may comprise materials providing an increased third-order non-linearity, materials that are photo-sensitive material(s), or are rare earth material(s).
The use of polymer(s) as background material for the fibres allows potentially cheap and very flexible fabrication of the fibres covered by the present invention.
The use of the above mentioned materials for the core and/or cladding regions allows the realization of various applications, such as fibre laser, amplifiers, wavelength converters, optical switches etc. Higher order (non-linear) effects may be used for e.g., soliton communication or more generally in applications, where non-linear effects are influencing the propagation properties of signals in optical communication systems. This also includes realisation of components for optical signal processing and for switching. Especially for applications for fibre lasers or fibre amplifiers, the dopant in the core or the cladding may be e.g., a rare-earth dopant adapted to receive pump radiation and amplify radiation travelling in the core region. The dopant may also be a light sensitive dopant, such as Germanium. In that situation, the dopant may be used for e.g. optically writing a grating in the fibre or core region. Of particular interest is the use of photosensitive materials to allow writing of 1D gratings in the longitudinal direction of the fibres. Fibres with such gratings, combined with the large mode area, are very attractive for high power fibre lasers.
For a range of applications, it is desirable to control the polarization of light guided through the fibre as well as the dispersion. The present fabrication techniques of micro-structured fibre make it difficult to completely eliminate asymmetries in the fibre cross-section. As those skilled in the art will recognize, this means that the fundamental mode of micro-structured fibres will have two nearly degenerate polarization states. For micro-structured fibres, where polarization effects are sought eliminated (the fibres are intended to have a low birefringence), we will note the two polarization states as substantially non-degenerate. For micro-structured fibres where, on the other hand, polarization effects are desired, the non-degeneracy may be enhanced such that the birefringence can reach levels of 10xe2x88x925 and even higher, such as of at least 10xe2x88x923. We will note such fibres as having a fundamental mode consisting of two substantially, non-degenerate polarization states. For a long range of applications, such as e.g. for high precision lithographic systems, it is desired to have fibres with high birefringence. Therefore, the different aspects of the present invention cover embodiments with fibres that guide light in two substantially, non-degenerate polarization states. To quantify the splitting of the polarizations states, the invention covers preferred embodiments, where the fibre birefringence is at least 10xe2x88x925, such as at least 10xe2x88x924, such as at least 10xe2x88x923.
The present invention covers embodiments wherein shape of the core region is essentially circular or elliptical, and/or wherein the core region has a substantially two-fold symmetry, which may be obtained from arrangement of the core features in a substantially two-fold symmetric manner. Furthermore, the invention covers embodiments wherein the core region and/or cladding region has substantially 180 degree rotational symmetry in the fibre cross-section.
To control the degree of birefringence, it is preferred to have fibres with a core region having either small or large degree of asymmetry. This may be obtained by a core region, in which the core features have a non-circular symmetric shape in the fibre cross-section. It may also or alternatively be obtained either by positioning of the core features in an asymmetric manner, in an otherwise symmetric core region, or by having an asymmetry in the actual shape of the core region. Naturally, combinations of the afore-mentioned cases may also be employed. By asymmetry is here meant a deviation away from a circular symmetric shape or away from a quadratic, a hexagonal or a symmetric, higher order polynomial shape. The present invention, therefore, covers preferred embodiments with the above-described manners of applying asymmetry to the core region. However, the invention may also cover embodiments in which the shape of the core region is substantially rectangular in the fibre cross-section. Fibres according to the present invention may often have a solid overcladding surrounding the micro-structured cladding and core regions. Typically, this overcladding will consist of silica having a higher refractive index than the micro-structured cladding region in order to strip off cladding modes.
In some of the above-described embodiments, it has been assumed that the core and cladding features have a lower refractive index than the core and cladding material (the background material of these regions). The reason for this is that micro-structured fibres commonly are fabricated such that the cladding features are voids such as air holes, hence the features have a lower refractive index than the surrounding background material. It is, however, important to notice that the inventions and ideas described in this application are also valid and may be utilized in the case of high-index features.
The present invention also covers fibres, where the cladding features are placed in concentric manners, such as in concentric annular regions surrounding the core.
The present invention also covers fibres, where the cladding features are increasing in size with respect to their distance to the centre of the core.